Use function/method defaults instead of argparse defaults

Question

When using argparse, I've not found an elegant/DRY way to use the function/method defaults instead of the defaults passed by argparse.

For example, I have foreign code I am loathe to modify. How do I tell argparse (or elegantly handle after argparse) to use the function defaults if the user does not pass in a clear preference on the command-line?

import argparse

def foreign_func(fav_color="blue"):
    print(fav_color)

clparser = argparse.ArgumentParser()
clparser.add_argument(
    "--color",
    default=None,
    help="Enter color")
clargs = clparser.parse_args()

foreign_func(fav_color=clargs.color)

This will print 'None', instead of "blue"

My default approach is something like:

if clparser.color:
    foreign_func(fav_color=clargs.color)
else:
    foreign_func()

but this seems clunky, especially if there are multiple command-line options.

EDIT: Hi folks, thank you for the fast feedback. To clarify, although it would be nice for argparse to display 'blue' in it's help, that's not my goal.

I'm looking for:

my_prog --color=red
>>> red

my_prog 
>>> blue

With the code above, the program is outputting 'None', not "blue"

Answer 1

You can use inspect to get the function's default parameter value, and use that as the default in the argparse code.

import argparse
import inspect

def foreign_func(fav_color="blue"):
    print(fav_color)

clparser = argparse.ArgumentParser()
clparser.add_argument(
    "--color",
    default=inspect.signature(foreign_func).parameters['fav_color'].default,
    help="Enter color")
clargs = clparser.parse_args()

foreign_func(fav_color=clargs.color)

Note that this will behave a bit oddly if there is no default in the function (if it's a required parameter). In that case, the default value will be the inspect._empty sentinel value, rather than an actually meaningful value. But your code seems to be assuming that the parameter is always optional, so I'm going with that too.

Answer 2

You can use a dictionary to specify your arguments by name; if a name is missing, that parameter will use its default. Example:

def func(a=1,b=2):
    print("a=",a,"b=",b)

func()
func(**{})
func(**{'a':10})

will print

a= 1 b= 2
a= 1 b= 2
a= 10 b= 2